1. Field of the Invention
The present invention relates to a method for forming a thin film transistor and, more particularly to a method for forming a thin film transistor, which can obtain good characteristics at a low temperature.
2. Description of the Related Art
A thin film transistor is used in a device having an large area and a transmittancy, which cannot be formed on a monocrystalline wafer, such as a liquid crystal display. In such a device, glass material having the transmittancy and the low cost is used as a base material. Since a melting point of glass material is low, glass material cannot be applied to the conventional method for forming the transistor on the monocrystalline wafer. For example, in the case where the conventional monocrystalline wafer is used, a gate insulating film is generally formed by thermally oxidizing a silicon wafer. However, in this thermal oxidation method, since a high temperature treatment such as 1000.degree. to 1300.degree. C. is performed, this thermal oxidation method cannot be applied to the case that base material is glass having a low melting point.
In other words, all processes for forming the thin film transistor must be performed at a low temperature as compared with the method for forming the transistor on the monocrystalline wafer. In addition, peculiar characteristics of amorphous or polycrystalline semiconductors, which are different from the characteristics of a monocrystalline semiconductor, must be added to the thin film transistor. Therefore, if the conventional manufacturing method is directly used in forming the thin film transistor, the characteristics are largely lowered. Particularly, the method for forming the gate insulating film is an important point. As a method for forming the gate insulating film at a low temperature, there is known the method in which silicon oxide or silicon nitride is deposited by CVD (Chemical Vapor Deposition). A silicon nitride gate insulating film can obtain a good breakdown voltage. However, a characteristic of a surface boundary between the silicon nitride gate insulating film and a thin film semiconductor layer is deteriorated. Particularly, the mobility of the positive hole is low, and the value of the mobility of the hole is lower than that of an electron by one digit. On the other hand, a characteristic of a surface boundary between the silicon oxide gate insulating film and the thin film semiconductor layer is relatively good. However, there is a disadvantage in that the breakdown voltage of the silicon oxide film is inferior to that of the silicon nitride film.